MXS-1507: Retry interrupted queries in transactions

As the current query was added to the transaction log before it finished,
the m_current_query contained a duplicate of the latest transaction log
entry. To correctly log only successful transactions, the statement should
be added only after it has successfully completed. This change also
removed the unnecessary cloning that took place when the statement was
added to the log before it finished.

With the fixed transaction logging, the value of m_current_query can be
stashed for later retrying while the replay process is happening. If the
replay completes successfully and the checksums match, the interrupted
query is retried.

Also added a clarifying comment to can_retry_query to explain why a query
inside a transaction cannot be retried.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -140,6 +140,7 @@ public:
     Trx                     m_trx; /**< Current transaction */
     bool                    m_is_replay_active; /**< Whether we are actively replaying a transaction */
     Trx                     m_replayed_trx; /**< The transaction we are replaying */
+    mxs::Buffer             m_interrupted_query; /**< Query that was interrupted mid-transaction. */
 
 private:
     RWSplitSession(RWSplit* instance, MXS_SESSION* session,
@@ -194,6 +195,12 @@ private:
 
     inline bool can_retry_query() const
     {
+        /** Individual queries can only be retried if we are not inside
+         * a transaction. If a query in a transaction needs to be retried,
+         * the whole transaction must be replayed before the retrying is done.
+         *
+         * @see handle_trx_replay
+         */
         return m_config.delayed_retry &&
                m_retry_duration < m_config.delayed_retry_timeout &&
                !session_trx_is_active(m_client->session);